Well, there are some airborne pulse EMs and magnetometers. Both expensive and complex to use but very easy to hire someone ready to go--if you have enough bucks to spend on the project.

But with any method you have to have sort of ideas of size versus depth versus composition of target versus possible flight heights (flat desert versus forested mountains) versus ground matrix composition, etc.

In other words before you even get a quote, they will need to have some available parameters. It may turn out that some equipment may work, others may not. General idea though, it will have to generate a relatively big anomaly to be detected.